The full complement of genes and pathways required for mammalian innate resistance to a defined pathogen has not previously been examined using germline mutagenesis. Yet it is likely that many of the key elements of the innate immune response will be missed by other approaches, given the complexity of interactions that occur between cells of the immune system in vivo. In recent years we have made substantial progress toward defining the genes that protect mice against an early lethal outcome when they are inoculated with the beta-herpesvirus mouse cytomegalovirus (MCMV). By screening 12,170 G3 germline mutant mice from 2028 pedigrees, we have collected a total of 2 dominant germline mutations and 29 recessive germline mutations that markedly impair the innate immune response to MCMV. 7 additional mutations that impair MCMV resistance were identified by cross-screening mice identified in other screens for innate immunodeficiency, so that 38 mutations have been collected in all. We have begun to map, exclude allelism, cross-phenotype for susceptibility to other microbes, and positionally clone each of these mutations (and in 8 instances, have identified the defects already). Based on comparative estimates of saturation mutagenesis in which lethality is taken as an endpoint, we believe that approximately 11% of all host resistance genes have been modified in our primary screen, and calculate that about 280 genes encode proteins with non-redundant functions in resistance to MCMV. Other germline genetic studies have, to date, implicated only 23 genes in innate resistance to MCMV infection; hence, most of the MCMV "resistome" is still unknown. In this proposal, we describe a means of identifying many additional non-redundant resistance genes. It is expected that some will be known participants in the innate immune response, but that many will be novel. This will permit us to develop a well-defined picture of the defensive processes that are set in motion by beta-herpesvirus infections. Because it is clear that innate immunity is degenerate in that individual proteins frequently offer broad defense against bacterial, viral, and fungal pathogens, these studies will yield insight into innate immune mechanisms at large. [unreadable] [unreadable] [unreadable]